Question & Answer Session
Danielle, age ?, from Colchester in England, UK asked "My teacher has asked us to find out about sharks skin, and if they have scales?"
Well Danielle, the answer is no, sharks don't have scales but they have something different instead - Dermal Denticles, literally "skin teeth" rows and rows of tiny teeth made out of dentine, the same stuff as teeth!
The dermal denticles are embedded in the skin and face backwards and are all aligned (this is why sharks feel smooth when you stroke them from head to tail but rough when you stroke them backwards. Many aquariums have petting tanks where you can pet small harmless sharks e.g.nurse sharks, dogfish etc). They often also have the same shape as the teeth of the shark (though not all).
The denticles do several things :
- Protect the shark
- Help it swim through the water more easily.
The dermal denticles help reduce the friction between the shark and the water (they help it slip through the water more easily).
When water flows over a smooth object it must be moved out of the way by the object causing friction. The water moving close to the surface of the object doesn't always move smoothly. Even a smooth object has little imperfections that cause the water to create eddies and swirl around near to the object (this area is called the boundary layer and there's a lot of friction and resistance there). As the water swirls it rubs against itself and creates friction rather than moving around the object in nice smooth lines.
The sharks dermal denticles all line up an although the surface of the shark is rough the roughness lines up and helps keep the water moving over the surface of the shark (in the boundary layer) moving in nice smooth straight lines. So funnily enough a rough object can move through the water with less effort than a smooth one.
This effect has been duplicated by engineers in experiments to try to control the boundary layer flow on airplanes and ships so they can go faster with less energy.
I hope this answers the question for now, later on there will be some diagrams and a page just about shark skin.
Here are a few links for you :